rustbuild: don't require network for vendoring libtest

[Keruspe]

Fixes #79218

[rust-highfive]

r? @Mark-Simulacrum

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[Keruspe]

Hmm, it seems --offline isn't actually offline :(

[Keruspe]

I have a better fix currently testing, as --offline isn't offline at all, still tries to sync crate.io

[Keruspe]

So, trying with --locked made the actual error pop up.

cargo vendor --offline doesn't help as it's not cargo vendor that is at fault. it's the Cargo.lock generation.

[Keruspe]

I'll reopen another PR once I actually get a working solution.